Method of annealing sheet steel



Patented Jan. 12, 1926.

GUERNEY H. COLE, OF FOREST HILLS BOROUGH,

INGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENN-SYLVANIA.

METHOD OF ANNEALING SHEET STEEL.

' No Drawing.

T (1 whom it may concern:

Be it known that I, GUERNEY H. COLE, a citizen of the Vnited States, anda resident of F orest Hills Borough, in the county of Allegheny andState of Pennsylvania, have invented a new and useful Improvement inl\[ethods ol' Annealing Sheet Steel, of which the following is aspecification.

T his invention relates to annealing metals, such as steel, siliconsteel, etc, it being especially directed to an improved method ofre-annealing electrical sheet metal, such as is ordinarily used inmaking transformer cores.

Steel sheets for transformer cores are generally annealed to a pointwhere molecular rearrangement takes place, and the material so treatedis then punched or cut to the desired form, after which the metal isreannealed in order to remove the strains set ,up during the punching orcutting process and to reduce the core loss of the metal, it being amongthe objects of this invention to devise a method for re-annealing suchsheet material which is simple and economical.

Hitherto, it has been the custom to reanneal material, such as sheetsilicon steel, in a receptacle of suitable kind, such as a box or potsealed from the atmosphere by a cover or other suitable means, and thematerial was heated to a temperature of 785 to 1000 C. for aconsiderable period of time. I Usually, the sheet metal to be annealedwas stacked in a large box on a car and a heavy cast iron or steel coverplaced thereon. All cracks were carefully sealed with sand and clay toprevent access of air. The material, was then placed in a furnace whereit was heated to about 800-S50 C., the heating requiring 36 to 48 hours.It was held at this temperature for about 8 hours, after which it wasallowed to cool to room temperature which required several days. It isapparent that the heat losses were large because of the necessity ofpenclrating the heavy cast iron cover, a large amount of labor wasnecessary to prepare-the steel for annealing, and much time was consumedin the process.

It was generally considered that the higher the temperature of annealingthe greater would be the improvement in the annealed material, providedoxidation thereof could be kept reasonably low. Precautions were takento prevent access of air,

Application filed December 21, 1920. Serial Nb. 432,336.

and the temperature used was as high as was economically possible. Manyprocesses utilized a temperature as high as 1100 C. or even higher inre-annealing. It will be seen that such methods had seriousdisadvantages in that the material was heated to a high temperature,necessitating the use of large amounts of fuel and requiring constantand careful supervision over the annealing process.

My invention obviates these disadvantages in a novel manner. I havediscovered that a great improvement in core loss may be attained withoutthe use of covers or other means for preventing access of air and I havealso found that the use of a low temperature, very much lower than anywhich had hitherto been considercdfavorable for re-annealing, producedremarkable results in improving the core loss of electrical sheet steel.I have found that heating the material with free access of air at atemperature about 100 C. less than the minimum temperature hithertoused, produced double the improvement in core loss of electrical steel,compared to that produced by the old methods, excellent results havingbeen obtained at temperatures below 645 C.

The analysis of the atmosphere surrounding the material being annealedhas shown that the ratio of oxygen to nitrogen in the fninace was nearlyas great as that usually found in normal air. It was also found that thepercentage of water vapor in the furnace atmosphere was very high, but Iam inclined to believe that the water vapor present contributes butlittle to the improvement produced by my new method.

In the present case, there is no detrimental effect of carbon monoxideor carbon dioxide upon the material being annealed on account of the lowtemperature used, such temperature being too low to allow of sub-'stantial ionization of these gases and, therefore, prevents combinationof the same with the material being annealed. Therefore, in my method,it is not necessary to exclude the products of combustion from thematerial and, if a mufile furnace is used in the annealing, leakage ofproducts of combustion into the muflle on account of cracked orburnt-out casings is not detrimental to the material. It is seen,therefore, that perfect and delicate control of the various factors isunnecessary with my method.

PENNSYLVANIA, ASSIGNOR TO WEST- Although I prefer to use atmospheric airin my method, I may substitute therefor a sufficient amount of pureoxygen or a substance capable of furnishing the desired amount of oxygento the atmosphere surrounding the sheet material. I attempt to avoid astrong current of air through the material being annealed, since toomuch oxygen may cause excessive oxidation of the surface of the annealedmetal and thus lower the electrical efliciency thereof. However, I havefound that a' thin film of reddish oxide does not materially decreasethe efiiciency of the steel for electrical purposes.

My method, as practiced with the use of a continuous furnace of themuflle type, is as follows: The furnace, which may be of considerablelength, having a double muiile with tracts to allow cars to move in eachof the compartments in opposite directions, is heated by any suitablemeans, such as gas or oil, to a temperature of approximately 700 C.Sheet steel is stacked on cars and slowly passed through the furnace,the end doors of which may be partly open in order to permit free accessof air. ,If'it is found that -too much air is being circulated, thedoors of the furnace may be more or less completely closed until it isfound that sufficient air is entering to properly complete theannealing, and such adjustment will be permanent for any one set ofconditions involved. In a large furnace it is preferable to keep thedoors completely closed. The sheet material is allowed to pass throughthe furnace at such a rate that it reaches a temperature of about 700 inabout 7 to 13 hours, such temperature being held for about 3 to 6 hours.

I have described above the use of a mufile furnace with my new method ofannealing, but it is to be understood that my method is equallyapplicable to furnaces of other types and, although I have designated700 C. as the preferable temperature at which optimum results areobtained, it is to be understood that other temperatures, even as highas 7 50, give a marked improvement over the results obtained by formermethods. Lower temperatures are also beneficial and I have found that atemperature even as low as 600 C. will give excellent results. At thetemperature of 700 C., which I prefer to use, the improvement in thecore loss by re-annealing ranges from 15 to 25% even if the bestcommercial method was followed in the original annealing, whereas, byutilizing former methods, the best improvement that was ordinarilyobtained was 642%. A teme rature of 645 C. gave an improvement 0 about15%.

It. will be seen that, by the use of my new method, lower temperaturesmay be used with better improvement in the steel 'for electricalpurposes; not only is there great economy of fuel and labor effectedbecause of the lower temperature and shorter time necessary, but thewear on the furnace is more than proportionately decreased and itscapacity increased. Since the presence of air is beneficial, it isunnecessary to take care that the furnace is air-tight nor to expendtime and labor in making the annealing boxes air-tight. The expense offurnishing and replacing boxes and covers is eliminated.

I Having thus fully described my said in-.

vention, what I claim as new and desire to secure by Letters Patent is:

1. The' process of heat treating sheets of magnetic silicon alloy steelwhich consists in heating said sheets to a temperature not exceeding 700C. in an atmosphere containing substantial amounts of oxygen and thencooling.

2. The process of heat treating sheets of magnetic silicon alloy steelwhich consists in heating said sheets to a temperature not exceeding 700C. in an atmosphere containing oxygen in approximately the same ratio tonitrogen'asexists in air and then cooling.

3. The process of heat treating sheets of magnetic silicon alloy steelwhich consists in heating said sheets to a, temperature of about 700 C.in an atmosphere contain ng substantial cooling.

4. The process of heat treating sheets of magnetic silicon alloy steelwhich consists in heating said sheets to a temperature of about 700 C.in an atmosphere containing oxygen in approximately the same ratio tonitrogen as exists in air and then cooling.

5. The method of heat treating sheets of magnetic silicon alloy steelwhich consists in heating said sheets in an atmosphere containingsubstantial amounts of oxygen and water vapor and then cooling.

6. The method of heat treating sheets of. magnetic silicon alloy steelwhich consists in heating said sheets to a temperature of about 700C. inan atmosphere containing substantial amounts of oxygen, maintaining saidsheets at said temperature for a period of time and then cooling.

7. The method of heat treating sheets of magnetic silicon alloy steelwhich consists in heating said sheets to a temperature of about 700 C.in an atmosphere containing substantial amounts of oxygen and watervapor, maintaining said sheets at said temperature for a period of timeand then coolmg.

In testimony whereof, I have hereunto subscribed my name this 13th dayof Decem ber, 1920.

GUERNEY H. COLE.

amounts of oxygen and then-

